When you start losing weight your body response is programmed to first of all go into level 1 survival mode, which is to slow down your thyroid which can cause fatigue. This process is so that if your reduced food intake is due to a famine, you will conserve energy for important bodily functions. Check your body underarm temperature before getting out of bed in a morning. It should be at least 36.6C if your thyroid is working correctly. If not check with your doctor.